The present invention relates to borehole logging, and more particularly to borehole logging methods and tools having one or more extendable and retractable outrigger arms attached to the tool. Such arms may be used for determining the diameter of the borehole through which the tool is moving, for centering or eccentering the tool in the borehole, and/or for carrying tool components outwardly from the main body of the tool itself.
To provide for passage through wellbores, casing, tubing, and so forth, of different diameters, such outrigger arms are usually fabricated as cantilevered arms or as outwardly-bowed, stiff, flexible, springs. In the more common bowspring outrigger arm arrangement, both ends of each arm are anchored to the main tool body. At least one end of each arm is then allowed to shift or to slide axially along the tool whenever the effective diameter of the arm changes in response to changes in the diameter of the borehole. This means, of course, that the exact axial position or relationship of the arm relative to the tool will change according to the effective arm diameter. In some tools and applications, this can be a distinct disadvantage.
For example, in many casing-collar locator tools, pairs of permanent magnets are carried on the outrigger arms to produce a magnetic bucking field adjacent the steel casing in a borehole. Centered beneath these magnets is a coil which detects changes in the magnetic bucking field as the tool passes casing collars while moving through the casing. This very accurately determines the exact depth of the tool in the wellbore. However, since the exact axial positions of the arms relative to the tool are a function of the radial extension of the arms, there will usually be but only one effective diameter for which the magnets are positioned exactly over the centerline of the coil. At other diameters, the configuration is less than optimal.
For certain borehole tools having radially extendable and retractable outrigger arms, therefore, a need exists for a displacement control mechanism for regulating and controlling the axial displacement of each of the outrigger arms relative to the tool such that a defined relationship will be produced for each effective arm extension from the tool.